This invention relates to a process for preparing subpigmentary titanium dioxide with increased photostability through doping, in which a hydrolyzable titanium compound is subject to a two step ripening or aging treatment in a tank or vessel, and to an optional post-treatment. The invention further relates to subpigmentary titanium dioxide containing a doping substance (dopant) for improving its photostability.
The term "subpigmentary titanium dioxide" refers to titanium dioxide having a particle size in the range from about 10 to about 100 nm. Titanium dioxide subpigmentary particles have little or no light scattering properties because of their size, and are made by processes very different from those used to manufacture pigmentary titanium dioxide. Subpigmentary titanium dioxide, which is a recent development, is used as a transparent UV absorber, e.g., in paints, glazes, plastics and cosmetics. In metallic paints, subpigmentary titanium dioxide can be used to achieve special optical effects (e.g. "down flop" optical effects, see Panush, U.S. Pat. No. 4,753,829). Elfenthal et al., U.S. Pat. No. 5,215,580 describes a process for preparing subpigmentary titanium dioxide in which the particle size of the subpigmentary titanium dioxide can be influenced via addition of a colloidal tin oxide prepared in a special process.
Titanium dioxide pigments, having a physical minimum size greater than 100 nm, can be characterized by their scattering power for visible light. It has been long known to use titanium dioxide as a white pigment (particle size predominantly in the range of 200 to 500 nm) in paints, fibers, plastics and other systems. It is also known that such use can lead to undesired reactions, which cause disintegration of the medium surrounding the titanium dioxide particle. Such reactions are initiated by ultraviolet (UV) irradiation [see for example, H. G. Volz, G. Kampf, H. G. Fitzby, A. Klaeren, ACS Symposium Ser., 151 (1981) 163]. In this decomposition a catalytic effect is attributed to the titanium dioxide. The reactions underlying this decomposition occur predominantly at the surface of the pigmentary titanium dioxide particle. The influence of UV radiation on the photostability of a pigment can be described, for example, by means of a test for greying.
Since subpigmentary titanium dioxide, due to its much smaller particle size, has a surface area approximately 10 to 20 greater than times that of pigmentary titanium dioxide, subpigmentary titanium dioxide has a higher photoactivity or a smaller so-called light stability.
It is known to post-treat pigmentary titanium dioxide particles in order to lower photoactivity. The customary post-treatment is to coat the particles with oxides of silicon, zirconium and aluminum (see, for example European Patent 393,857-A1; European Patent 430,424-A2; or Japanese Patent 02-194063). For many applications, however, such a post-treatment when used with subpigmentary titanium dioxide does not result in sufficient photochemical stabilization.
Furthermore, in the preparation of titanium dioxide pigments according to the sulfate process it is known to add certain substances before calcining in order to decrease the photoactivity. In subsequent thermal treatment a portion of the precipitated metal ions diffuse into the interior of the crystals of the pigmentary titanium dioxide particles. In the case of chloride process - titanium dioxide pigments, dopants which increase the light stability or decrease the photoactivity are directly deposited intracrystallinely in the titanium dioxide particles by introducing the dopants into the oxygen-titanium tetrachloride burner.
In addition, with traditional titanium dioxide pigments it is known to precipitate substances, which can function as dopants for reducing photoactivity, in a one step process, onto the titanium dioxide pigment, and subsequently to anneal the pigment. Since this process involves a diffusion-controlled phenomenon, tempering must be carried out at high temperatures, and in some cases for extended times (German Patent No. 25 45 243; 0.1-10 hours).
It is not possible to use procedures of the above types to treat subpigmentary titanium dioxide, since such heat conditions would cause particle growth of the subpigmentary titanium dioxide and the creation of undesired normal-sized pigment particles. Such a treatment is also excluded by economic considerations in view of the larger surface area involved with subpigmentary particles and the resulting need for much larger amounts of dopant.
As used herein, the terms "intracrystalline inclusion" and "bulk doping" are understood to refer to a substantially homogeneous distribution of the foreign substance (i.e., dopant) in the internal regions of the crystalline titanium dioxide particles. This is to distinguish the term from "interstitial inclusion", which is a surface treatment on, or between, titanium dioxide pigment particles.
Prior to the present invention, attempts to distribute a dopant intracrystallinely in subpigmentary titanium dioxide particles have not been successful.